/*
 * Copyright (C) 2016 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.CollectPreconditions.checkRemove;

import com.google.common.annotations.GwtCompatible;
import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * Similar to {@link TransformedIterator}, this iterator transforms a backing iterator. However,
 * rather than enforcing a one-to-one mapping, each element in the backing iterator can be
 * transformed into an arbitrary number of elements (i.e. a one-to-many mapping).
 *
 * @author James Sexton
 */
@GwtCompatible
abstract class MultitransformedIterator<F, T> implements Iterator<T> {
    final Iterator<? extends F> backingIterator;

    private Iterator<? extends T> current = Iterators.emptyIterator();
    private Iterator<? extends T> removeFrom;

    MultitransformedIterator(Iterator<? extends F> backingIterator) {
        this.backingIterator = checkNotNull(backingIterator);
    }

    abstract Iterator<? extends T> transform(F from);

    @Override
    public boolean hasNext() {
        checkNotNull(current); // eager for GWT
        if (current.hasNext()) {
            return true;
        }
        while (backingIterator.hasNext()) {
            // checkNotNull the assignment, so that current is null even if the exception is caught
            checkNotNull(current = transform(backingIterator.next()));
            if (current.hasNext()) {
                return true;
            }
        }
        return false;
    }

    @Override
    public T next() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }
        removeFrom = current;
        return current.next();
    }

    @Override
    public void remove() {
        checkRemove(removeFrom != null);
        removeFrom.remove();
        removeFrom = null;
    }
}
